When potentials are to be measured, for example, on the skin of a patient and the useful signal contained in these potentials lies in the mV range, as this may be the case in an electrocardiogram (ECG) or an electromyogram (EMG), the following problems arise.
As the body of the patient is surrounded by electric fields, potentials form due to capacitive coupling on the skin of the patient. This effect can generally be described as the body being coupled capacitively, particularly to a 230V/50 Hz-alternating current voltage field, which is caused by power supply sources located in the surrounding area of the patient. For the sake of safety, the patient is not coupled to a uniform surrounding area ground, since this would cause a considerable risk to the patient.
In addition, the problem arises that a measuring device, with which the electrodes on the skin of the patient are connected, has to be galvanically separated from a surrounding area ground. This in turn results in the measuring device, with an internal ground, also being capacitively coupled to the surrounding area. The problem arises that the device ground lies at a potential, that is at a level that is not known and which generally differs from the potential of the patient.
In order to in this case at least achieve that the patient and the ground of the measuring device are at the same potential, or that at least a fixed potential difference is present between both, it is known to connect the device ground and the body of the patient with one another via an additional electrode. This is shown in FIG. 1. Since, however, the device ground and the patient may generally be at a different potential, because of the inhomogeneity of the surrounding fields, which arises from the different capacitive coupling to the surrounding area, a compensating current flows. This leads to a so-called common mode signal because of the impedance of the coupling to the patient via the electrode. This common mode signal is amplified by the amplifiers in the measuring device. When the useful signals actually to be detected with the measurement are very small, the common mode signal leads to the actual useful signal no longer being able to be resolved. In addition, the difficulty arises that the amplifiers have to be high dynamics amplifiers, so that the useful signal and the higher common mode signal overlaying same can be processed. Furthermore, a digital electronic analyzer arranged downstream has to provide a high number of bits per measured value in order to be able to process the large signals.
Furthermore, it is known from the state of the art of DE 29 26 165 A1 to subtract the mean value of the signals that are output by the amplifiers from the input signals of the amplifiers. In this case, however, there is also the problem that the common mode signal is not amplified, but nevertheless is output together with the useful signal at the output of the amplifier. When the useful signal is extremely small, this can lead to the level of the common mode signal and that of the amplified useful signal nevertheless being on the same order of magnitude, so that these cannot be separated from one another easily. Besides, there is the problem that the amplifiers and an electronic analyzer arranged downstream have to be adapted to further process the comparatively large common mode signal as well.